A conventional technique for separating active semiconductor regions along the upper surface of a silicon wafer is oxide isolation in which a thick insulating field region of silicon dioxide is grown laterally around the active regions. Along the edge of each active region in some devices, the field-oxide isolation region is in the shape of a bird's head which protrudes upward beyond adjacent parts of the oxide and silicon regions.
Electrical contact to selected parts of the active regions is made with a patterned conductive layer that overlies the oxide and silicon regions. The conductive layer is typically created by depositing a metal layer on the top of the structure and then removing undesired parts of the metal. As a result, there are at least two changes in height along the resulting upper surface. One height change extends from the top of the metal portion above the bird's head to the top of the adjacent metal. Another height change is at the lateral edges of the remaining metal.
If another patterned metal layer is to be used, an insulating layer is first deposited on the upper surface of the structure. The profile of the top of the insulating layer generally reflects the profile of the first metal layer. For example, see R. Edwards et al, U.S. Pat. No. 3,962,779, which shows that the top of the insulating layer reaches a maximum height above the bird's head and drops to a minimum height in areas beyond the edges of the first metal layer. This surface roughness creates difficulty in providing good coverage with the second metal layer.
Surface unevenness in this type of structure can be reduced by removing some or all of the bird's head. Y. Hom-Ma et al disclose one way of doing this in U.S. Pat. No. 4,025,411. In Hom-Ma et al, a layer of photoresist is formed on the bird's head and on the adjacent parts of the structure in such a manner that the upper photoresist surface is largely planar. The structure is then subjected to a sputter etching in which the photoresist and silicon dioxide are attacked at close to the same rates. Consequently, the bird's head is removed without removing adjacent parts of the oxide isolation region. While surface roughness is reduced, the etching must be controlled quite carefully to avoid penetration into the active regions.
Hom-Ma et al also use a similar technique in planarizing an insulating layer formed on a patterned metal layer lying on a flat surface. No bird's head is present. A polymeric layer having a largely planar upper surface is formed on the metal and on adjacent parts of the insulating layer. The structure is then ion milled so as to bring the planar surface downward until the metal is exposed. Although this technique may be useful in some limited applications, it does not extend to the situation in which the first metal overlies a bird's head and must be electrically separated from part of an overlying second metal layer.